Fix the real/imag part splitting of expressions involving erf, erfc or erfi
Series expansion for "_erfs" helper function at point I*oo

It does the job:

>>> gruntz((I*erfi(I*x) - I*erfi(I*(x - exp(-exp(x)))))*exp(x**2)*exp(exp(x)), x, oo)
-2/sqrt(pi)

>>> gruntz((I*erfi(x) - I*erfi(I*(x - exp(-exp(x)))))*exp(x**2)*exp(exp(x)), x, oo)
oo*I

>>> gruntz((erfi(x) - I*erfi(I*(x - exp(-exp(x)))))*exp(x**2)*exp(exp(x)), x, oo)
oo

The first one is just example 8.23 from Gruntz' thesis. With a "rewrite(erf)"
this reduces to "erf" of real argument and limit point oo, hence can be easily
done with the old code.